Jaundice is the most common clinical diagnosis in the newborn, with nearly two-thirds of all neonates displaying significant unconjugated hyperbilirubinemia. Phototherapy is the most often and widely used treatment to accelerate the excretion of unconjugated bilirubin and reduce the risk of bilirubin-related neurotoxicity. The broad, long-term objectives of the project are to elucidate the mechanism of phototherapy, to understand the role of 3-dimensional structure, chirality and hydrogen bonding in bilirubin conjugation, hepato-biliary transport and excretion. The project focuses on the importance of pigment conformation (shape) int he partitioning of photochemical processes and in the control of photoproduct development, in the expression of amphophilic properties, in binding to proteins involved in transport and in membrane transport, in hepatic conjugation and excretion, in anion formation and gallstone nucleation. The SPECIFIC AIMS of the project are: 1. To clarify and define the 3-dimensional structure(s) of bilirubin, its anions and photoisomers and their metabolites by spectroscopic and molecular dynamics methods; to determine the bilirubin binding site and conformation on human serum albumin and the pK of bilirubin in a variety of environments by C-NMR. 2. To design and prepare synthetic bilirubin analogs as molecular probes of the role of secondary structure and hydrogen bonding in photobiologic mechanisms, hepatic uptake, conjugation and excretion. 3. To study a newly discovered photoisomerization bilirubin reaction and evaluated its importance in phototherapy. 4. To elucidate the mechanism of bilirubin photocyclization to lumirubin, which, is thought to be principal form of bilirubin excreted during phototherapy, evaluated the influence of local environment (solvent, lipid, protein) on the promotion of photocyclization and chirality transfer, determine the structure of optically active lumirubin and characterize its autoxidation products by spectroscopic methods. 5. To investigate the specific molecular features of pigment molecules (conformation, chirality, hydrogen bonding) important in amphiphilicity and molecular recognition in normal hepato-biliary transport. 6. To understand the structural factors influencing differing rates of hepatic clearance of bilirubin photoisomers (lumirubin and Z/E isomers), especially the role of secondary structure and hydrogen bonding. The studies are directly relevant to the prevention of neurologic damage in the jaundiced newborn and to the understanding and diagnosis of liver disease. They aim to elucidate an important metabolic detoxification pathway and lead to improved methods for treatment of severe neonatal jaundice, the Crigler-Najjar Syndrome, and prevention of bilirubin encephalopathy.